Boiler tube failures are a major cause of forced shutdowns in fossil fuel power plants. As a result of various operational conditions such as heat, pressure, and wear over time, boiler tubes eventually begin to fail by developing circumferential and axial cracks, as well as experience wall thinning (through both erosion and corrosion). When a boiler tube begins to leak, for example, steam escaping through the leak is lost to the boiler environment. Unless the leak is discovered and repaired, the leak may continue to grow until the tube eventually ruptures, thereby forcing the utility operating the boiler to shut it down immediately. These failures prove to be quite expensive for utilities and, as such, early boiler tube leak detection methods are highly desirable.
To this end, there are several technologies available for nondestructive inspection of structure surfaces, including eddy current, magnetic particle, and dye penetrant techniques. In the case of remote field eddy current inspection, the technique is susceptible to material property variations inherent within a material, thus resulting in signals that can either mask a defect or that can be mistakenly interpreted as a defect. Moreover, existing eddy current techniques cannot quantify and characterize any damage that is found. With respect to magnetic particle and dye penetrant techniques, both involve large amounts of chemicals and are not suited for high speed inspection of boilers due to the time required for chemical application and signal interpretation.
Another non-destructive technique that may be used for boiler tube inspection is ultrasonic testing. In ultrasonic testing, a transducer sends pulse waves into the surface of an object, and receives a return echo indicative of an imperfection. A coupling medium (e.g., liquid) is typically used to provide an effective transfer of ultrasonic wave energy between the transducer and the surface being inspected. In order to conduct an inspection at multiple angles with a single transducer, multiple passes are typically required. Alternatively, phased array ultrasonic sensors utilize a linear or two-dimensional array of ultrasonic transducers that are sequentially pulsed in sequence. Through superposition of individual wavelets, phased arrays provide the capability of steering the angle of the beam. Thus, the beam angle may be set by adjusting the timing of the individual pulses.
Notwithstanding the advantages offered by phased array ultrasonic sensors, tubes used in industrial boilers present a difficult challenge with respect to inspection, as the space surrounding the tubes (and thus access thereto) is typically very limited. In boiler systems, wall-loss is a major concern for small diameter (e.g., 1-2 inch) tubing, where the outside diameter of such tubes is not accessible. As a result, inspection from the inside of these tubes is often required. However, such tubes typically also have small radius bends (e.g., 5-6 inches) and are often swaged (tapered) and the ends thereof. These constraints in tube geometry make it difficult to implement, an effective, full-length inspection of the tubes as existing ultrasonic probes cannot traverse through the extreme bends and swages present therein.
Accordingly, it would be desirable to provide an improved probe for applications such as boiler tube inspection.